It is known that an aqueous dispersion of a copolymer obtained by emulsion polymerization of vinyl monomers can be used for forming tacky films or adhesive films. These films have poor adhesion to a surface of a material having small polarity, such as polyethylene, polypropylene, etc., and the shortage of adhesion has been covered by addition of a tackifier, such as rosin, rosin esters, etc., to the copolymer dispersion after polymerization. However, addition of tackifiers of this type has encountered a problem of serious reduction in cohesive force.
In order to improve adhesion of a film formed by the aforesaid aqueous copolymer dispersion, it has been proposed to conduct the emulsion polymerization in the presence of a thiol type chain transfer agent, e.g., t-dodecylmercaptan, 3-mercaptopropionic acid, etc., to thereby control the molecular weight of the produced copolymer. However, such a thiol compound containing a mercapto group, when used as a chain transfer agent, remains in the resulting copolymer and gives out an offensive smell. Although various proposals for removing the smell have been made, it is practically difficult, as is well known in the art, to completely remove the smell from the copolymer. In addition, it has been reported in Japanese Laid-Open Patent Application No. 228006/86 that a sulfur-containing compound used as chain transfer agent is bonded to the copolymer produced, resulting in reduction of durability of the copolymer.